Parathyroid hormone (PTH), a polypeptide of 84 amino acids produced by the parathyroid glands, regulates the levels of extracellular calcium via its actions on bone and kidney. The biosynthetic and secretory activity of the parathyroid gland is in turn regulated by extracellular calcium concentrations. Although the regulation of PTH secretion has been studied, little is known about the regulation of PTH biosynthesis at the molecular level. Previously, we showed that PTH is synthesized by way of two successive proteolytic cleavages of a larger precursor of 115 amino acids, pre-proparathyroid hormone (pre-proPTH). We also found that translational and post-translational processing of pre-proPTH were not regulated by calcium, suggesting that regulation must occur at the level of transcriptional and/or post-transcriptional/pre-translational processes. To study the regulation of these processes, we have prepared, cloned and sequenced a cDNA complementary to the mRNA coding for pre-proPTH. Utilizing this cloned cDNA, I have developed sensitive hybridization techniques for the measurement of cellular concentrations of this mRNA (PTH mRNA), and of the precursors of PTH mRNA under various in vitro and in vivo conditions of extracellular calcium. I propose to use these specific hybridization techniques to elucidate the regulation of PTH mRNA biosynthesis and metabolism by calcium in bovine parathyroid gland slices in vitro and in rat parathyroid glands in vivo. Such studies of the regulation of PTH biosynthesis have relevance to hyperparathyroidism, a common disease, inasmuch as it is characterized by abnormal regulation of parathyroid gland activity.